Oscillator circuit

ABSTRACT

An oscillator circuit may include a multivibrator for generating an oscillator signal, a supply circuit having a first, second and third current path, and a current mirror for mirroring a current through the second current path into the first current path, the third current path and a current path of the multivibrator. A first transistor in the first current path is operated in weak inversion and saturation on the basis of a first gate voltage. A second transistor in the second current path may be operated in weak inversion and saturation on the basis of the first gate voltage. A third transistor in the second current path may be operated in strong inversion and in the linear region on the basis of a second gate voltage. A fourth transistor in the third current path may be operated in strong inversion and in saturation on the basis of the second gate voltage.

The present application relates to an oscillator circuit and to a methodfor operating an oscillator circuit.

Oscillator circuits are used in diverse electronic circuits. In thisregard, by way of example, an oscillator signal having a substantiallyrectangular signal waveform that is generated by an oscillator circuitcan be used to control switching processes in a digital circuit.

For some areas of application it is desirable to use an oscillatorcircuit having a lower power consumption, in order thus to enable forexample an efficient realization of the oscillator circuit as part of anintegrated circuit. Examples of such areas of application are integratedsensor devices, integrated microprocessors or devices for controllingtransitions between energy saving operation and normal operation of anelectronic apparatus.

In order to enable a low power consumption of an oscillator circuit, itis known to use a multivibrator based on a band gap reference circuit,as described e.g. in U.S. Pat. No. 6,870,0433 B2. In this case, however,resistors having a high resistance in the region of 10 MΩ are requiredfor the band gap reference circuit. An implementation of these resistorshaving a high resistance by means of polysilicon structures typicallymeans a high area requirement in the resulting integrated circuit.

In “Low-power CMOS relaxation oscillator design with an on-chip circuitfor combined temperature-compensated reference voltage and currentgeneration”, by Yuchi Ni, Electrical and Computer Engineering Master'sTheses. Paper 127. http://hdl.handle.net/2047/d200049099 (2014), for anoscillator circuit on the basis of a band gap reference circuit and amultivibrator, it is proposed to realize a resistor having a highresistance in the band gap reference circuit by means of a MOStransistor (MOS: “Metal Oxide Semiconductor”). However, the proposedoscillator circuit requires a multiplicity of current paths in order toprovide a suitable gate voltage for the MOS transistor, which in turn isassociated with an increased area requirement in the integrated circuitand an increased power consumption.

It is therefore an object of the present invention to providetechnologies which enable an efficient implementation of an oscillatorcircuit with a low power consumption.

An oscillator circuit according to claim 1, an integrated circuitaccording to claim 13 and a method according to claim 14 are provided inaccordance with the present application. The dependent claims definefurther embodiments.

In accordance with one embodiment, an oscillator circuit is thusprovided. The oscillator circuit comprises a multivibrator forgenerating an oscillator signal. The multivibrator comprises at leastone current path. Furthermore, the oscillator circuit comprises a supplycircuit having a first current path, a second current path and a thirdcurrent path. A first field effect transistor is provided in the firstcurrent path. The first field effect transistor is configured to beoperated in saturation on the basis of a first gate voltage. A secondfield effect transistor is provided in the second current path. Thesecond field effect transistor is dimensioned in a manner deviating fromthe first field effect transistor, e.g. is provided with a greaterchannel width, and is likewise configured to be operated in saturationon the basis of the first gate voltage. In some embodiments, the firstfield effect transistor and/or the second field effect transistor are/isconfigured to be operated in weak inversion and saturation on the basisof the first gate voltage. Furthermore, a third field effect transistoris provided in the second current path. The third field effecttransistor is configured to be operated in the linear region on thebasis of a second gate voltage. A fourth field effect transistor isprovided in the third current path. The fourth field effect transistoris configured to be operated in the linear region on the basis of thesecond gate voltage. A current mirror is provided for mirroring acurrent in the second current path into the first current path, thethird current path and into the at least one current path of themultivibrator. A resistor having a high resistance in the region of 10MΩ can be realized by the third field effect transistor in an efficientmanner. The gate voltage of the third field effect transistor can inturn be generated via the fourth field effect transistor in an efficientmanner, without this necessitating a multiplicity of additional currentpaths.

In accordance with one embodiment, the at least one current path of themultivibrator comprises a fourth current path for charging and/ordischarging a capacitor of the multivibrator. The capacitor can beembodied on the basis of a fifth field effect transistor. The thirdfield effect transistor and the fifth field effect transistor can beembodied at least partly by common processes. Effects of processvariations can be compensated for in this way.

In accordance with one embodiment, the at least one current path of themultivibrator comprises a fifth current path for generating a referencevoltage for a comparator of the multivibrator. The fifth current pathcan comprise a sixth field effect transistor, which is configured to beoperated in strong inversion and in the linear region on the basis ofthe second gate voltage. The fifth field effect transistor and the sixthfield effect transistor can be embodied at least partly by commonprocesses. Effects of process variations can be compensated for in thisway. The comparator can be embodied on the basis of a single fieldeffect transistor.

In accordance with one embodiment, the oscillator circuit furthermorecomprises a high-voltage depletion-mode field effect transistor. Thehigh-voltage field effect transistor is coupled between a first supplyvoltage line for providing a supply voltage for the at least one currentpath of the multivibrator and for the first, second and third currentpaths of the supply circuit and a further supply voltage line. Via thehigh-voltage field effect transistor, a supply voltage for the variouscurrent paths can thus be derived from a further supply voltage on thefurther supply voltage line.

The oscillator circuit can be configured specifically for a low powerconsumption. In this regard, at room temperature a current consumptionof the oscillator circuit can be less than 100 nA. This enables anefficient implementation as part of an integrated circuit.

In accordance with a further embodiment, a method for operating anoscillator circuit is provided. The oscillator circuit can be configuredas described above.

In the method, a first field effect transistor in a first current pathis operated in saturation on the basis of a first gate voltage. In asecond current path, a second field effect transistor is operated insaturation on the basis of the first gate voltage. In some embodiments,the first field effect transistor and/or the second field effecttransistor are/is operated in weak inversion and saturation on the basisof the first gate voltage. The second field effect transistor isdimensioned in a manner deviating from the first field effecttransistor, e.g. is provided with a greater channel width. Furthermore,in the second current path, a third field effect transistor is operatedin strong inversion and in the linear region on the basis of a secondgate voltage. In a third current path, a fourth field effect transistoris operated in strong inversion and saturation on the basis of thesecond gate voltage. A current in the second current path is mirroredinto the first current path, the third current path and into at leastone current path of a multivibrator for generating an oscillator signal.

In accordance with one embodiment, a capacitor of the multivibrator ischarged and/or discharged via the at least one current path of themultivibrator.

In accordance with one embodiment, a reference voltage for a comparatorof the multivibrator is generated via the at least one current path ofthe multivibrator.

The field effect transistors used in the above embodiments can be formedby MOS transistors, e.g. n-channel enhancement-mode MOS transistors.

Further details of the embodiments mentioned and further embodiments aredescribed below with reference to the accompanying drawings.

FIG. 1 schematically illustrates a multivibrator of an oscillatorcircuit in accordance with one exemplary embodiment of the invention.

FIG. 2 schematically illustrates a supply circuit of the oscillatorcircuit.

FIG. 3 schematically illustrates characteristic curve profiles of fieldeffect transistors used in the oscillator circuit.

FIG. 4 shows by way of example a temperature dependence of a currentconsumption of an oscillator circuit in accordance with one exemplaryembodiment of the invention.

FIG. 5 shows by way of example a temperature dependence of the frequencyof an oscillator signal generated by an oscillator circuit in accordancewith one exemplary embodiment of the invention.

FIG. 6 shows a flow diagram for illustrating a method in accordance withone exemplary embodiment of the invention.

Exemplary embodiments of the present invention are explained in greaterdetail below with reference to the accompanying drawings. In this case,it goes without saying that the exemplary embodiments illustrated areintended to serve merely for elucidating implementation possibilities ofthe invention and should not be understood as a restriction thereof. Inparticular, features of different exemplary embodiments can be combinedwith one another. Furthermore, a description of one exemplary embodimentwith a multiplicity of features should not be interpreted to the effectthat all these features are necessary for implementing the invention. Byway of example, other exemplary embodiments could have fewer featuresand/or alternative features.

Exemplary embodiments illustrated below concern an oscillator circuit.The oscillator circuit can be provided in particular for fields ofapplication in which a low power consumption is desired. For example,the oscillator circuit can be provided for use as part of a compactintegrated circuit. Such an integrated circuit can implement for examplean autonomous sensor device, a device for switching between energysaving operation and normal operation of an electronic apparatus, amicroprocessor or a communication apparatus. In the exemplaryembodiments illustrated, the oscillator circuit is based on amultivibrator 10 which generates a rectangular oscillator signal OSC.The oscillator signal OSC can serve for example for digitallycontrolling switching processes in the integrated circuit. Signals usedin the multivibrator are generated by means of field effect transistorsoperated in different regimes. These regimes differ between operation inweak inversion and operation in strong inversion. Furthermore, adistinction is drawn between operation in saturation and operation inthe linear region. These modes of operation can be defined as followsfor a field effect transistor: in the case of operation in weakinversion a gate voltage of the field effect transistor is below athreshold voltage of the field effect transistor, whereas in the case ofoperation in strong inversion the gate voltage of the field effecttransistor is above the threshold voltage of the field effecttransistor. In the case of operation in saturation a drain-sourcevoltage of the field effect transistor is above a saturation voltage ofthe field effect transistor, wherein said saturation voltage in turndepends on the gate voltage and the threshold voltage. In the case ofoperation in the linear region the drain-source voltage of the fieldeffect transistor is below the saturation voltage of the field effecttransistor.

The multivibrator 10 is illustrated schematically in FIG. 1.

The multivibrator 10 illustrated in FIG. 1 is based on alternatecharging and discharging of a capacitor 20. A plurality of current pathsare provided for the operation of the multivibrator 10, in which currentpaths corresponding current sources 11, 11′, 12, 13 respectively supplya current Iptat. The current paths are embodied between a first supplyvoltage line, which supplies a high first supply voltage VDD, and asecond supply voltage line, which supplies a low second supply voltageVSS.

A current path supplied by the current source 11 has a switch 31 andserves for charging the capacitor 20. If the switch 31 is closed, thecapacitor 20 is charged with the current Iptat. As illustrated, thecapacitor 20 can be implemented by a MOS transistor 25 by virtue of agate terminal of the MOS transistor 25 being used as a first terminal ofthe capacitor 20, while a source terminal and a drain terminal of theMOS transistor are used as a second terminal of the capacitor 20. Thecapacitor 20 can thus substantially be formed by a gate oxidecapacitance of the MOS transistor 25.

A current path supplied by the current source 11′ has a switch 32 andserves for discharging the capacitor 20. If the switch 32 is closed, thecapacitor 20 is discharged by the current Iptat. The switches 31, 32 aredriven in a complementary manner by the oscillator signal OSC and aninverse signal OSCQ of the oscillator signal OSC, that is to say if theswitch 31 is closed, the switch 32 is open, and if the switch 31 isopen, the switch 32 is closed. A triangular voltage signal is generatedat the capacitor 20 as a result of the alternate charging anddischarging of the capacitor 20.

A current path supplied by the current source 12 comprises a comparator40, which brings about a comparison of the triangular voltage signalwith an upper comparator threshold and a lower comparator threshold. Theupper comparator threshold and the lower comparator threshold aredefined by means of a reference voltage Vptat.

A current path supplied by the current source 13 serves for operating anoutput stage 60 of the multivibrator 10. In order to obtain arectangular profile of the oscillator signal OSC, the output stage 60can have a saturating amplifier characteristic. A desired amplitude ofthe oscillator signal OSC can also be set via the output stage 60.

As illustrated, the comparator 40 can be formed by a single MOStransistor 45 having a threshold voltage Vth, to which MOS transistorthe current Iptat is fed at its drain terminal, the triangular voltagesignal is fed at its gate terminal and a further voltage is fed at itssource terminal, which further voltage alternates between two values inorder thus to define the upper comparator threshold and the lowercomparator threshold. The difference between these two values is definedby the reference voltage Vptat.

The reference voltage Vptat can be generated via a further MOStransistor 150, through which the current Iptat flows and which isoperated in strong inversion and in the linear region, such that itcorresponds to an ohmic resistor in terms of its effect. The MOStransistor 150 can be bridged via a switch 50 driven by means of theoscillator signal OSC. For the voltage at the source terminal of the MOStransistor 45 this results in a rectangular profile that alternatesbetween the reference voltage Vptat and VSS at the frequency of theoscillator signal OSC.

If the capacitor 20 is charged, the switch 31 is closed. The switch 32and the switch 50 are open. The source terminal of the MOS transistor 45is thus at the voltage VSS+Vptat, and the MOS transistor 45 is turned onif the triangular voltage reaches the value Vss+Vptat+Vth which definesthe upper comparator threshold. At this point in time a steep fall inthe voltage at the drain terminal of the MOS transistor 45 arises, whichleads to a falling edge in the oscillator signal OSC. As a result, theswitch 31 is opened, and the switches 32 and 50 are closed. Thecapacitor 20 is thus subsequently discharged, and the source terminal ofthe MOS transistor 45 is at the voltage VSS. The MOS transistor 45remains turned on as long as the triangular voltage is above the valueVSS+Vth which defines the lower comparator threshold. If the triangularvoltage reaches the value VSS+Vth, the MOS transistor 45 is turned off,which leads to a steep rise in the voltage at the drain terminal of theMOS transistor 45. This is in turn converted into a rising edge of theoscillator signal OSC.

The frequency of the oscillator signal OSC is thus defined precisely bymeans of the value of the current Iptat and the voltage Vptat. Thecurrent Iptat defines an edge steepness of the triangular voltagesignal, while the reference voltage Vptat defines an amplitude range ofthe triangular voltage signal. Said amplitude range is typically in therange of less than 100 mV, e.g. approximately 50 mV.

FIG. 2 schematically shows a supply circuit 100 which is used in theoscillator circuit for generating the current Iptat and the referencevoltage Vptat.

As illustrated, the supply circuit 100 has a first current path 101, asecond current path and a third current path 103, which are embodiedbetween the supply voltage line for VDD and the supply voltage line forVSS. Furthermore, FIG. 2 shows current paths of the multivibrator 10described in connection with FIG. 1, namely the current path forcharging the capacitor 20, designated by 104, the current path forgenerating the reference voltage for the comparator 40, designated by105, and the current path for operating the output stage 60, designatedby 106.

As illustrated, in the current paths 101, 102, 103, 104, 105 in eachcase on the side of the supply voltage line for VDD, MOS transistors111, 112, 113, 114, 115, 116 of the p-channel type are provided, whichrespectively serve as current source for the current path 101, 102, 103,104, 105, 106. What can be achieved by the use of the same gate voltageand an identical type of construction of the MOS transistors 111, 112,113, 114, 115, 116 is that the same current Iptat flows in each of thecurrent paths. The gate voltage used for the MOS transistors 111, 112,113, 114, 115, 116 is generated in the second current path 102 by thegate terminal of the MOS transistor 112 being connected to the sourceterminal of the MOS transistor 112. The current Iptat established in thesecond current path 102 is thus mirrored into the other current paths101, 103, 104, 105, 106. The value of the current Iptat, as explained ingreater detail below, is defined via further MOS transistors 121, 122,130, 140 in the first current path 101, the second current path 102 andthe third current path 103.

As illustrated, a first MOS transistor 121 of the n-channel type isprovided in the first current path 101, via which MOS transistor the MOStransistor 111 is connected to VSS. The MOS transistor 121 is operatedin the region of weak inversion and in saturation. This results in anexponential dependence of the current Iptat through the first MOStransistor 121 on a gate voltage Vngate of the first MOS transistor 121.For generating the gate voltage Vngate, the gate terminal of the firstMOS transistor 121 is connected to the drain terminal of the first MOStransistor 121.

In the second current path 102, a second MOS transistor 122 of then-channel type and a third MOS transistor 130 of the n-channel type areprovided, which are connected in series and via which the MOS transistor112 is connected to VSS. The second MOS transistor 122 is operated withthe same gate voltage Vngate as the first MOS transistor 121 andlikewise in the region of weak inversion and in saturation. This resultsin an exponential dependence of the current Iptat through the second MOStransistor 122 on the gate voltage Vngate. However, the second MOStransistor 122 is dimensioned in a manner deviating from the first MOStransistor. By way of example, the second MOS transistor 122 can have agreater channel width than the first MOS transistor 121. The ratio ofthe channel widths between the first MOS transistor 121 and the secondMOS transistor 122 may be e.g. 1:8. On account of the deviatingdimensioning, different current densities arise in the first MOStransistor 121 and the second MOS transistor 122.

The third MOS transistor 130 is operated in strong inversion and in thelinear region and serves as a series resistor between the second MOStransistor 122 and VSS. The resistance value provided by the third MOStransistor 130 is determined by a gate voltage Vb of the third MOStransistor 130 and may be in the range of more than 1 MΩ, for example 10MΩ.

The first current path 101 and the second current path 102 thus operatein the manner of a band gap reference circuit, wherein the currentIptat, on account of the exponential characteristic mentioned, is set toa value which, although dependent on the absolute temperature, theresistance value provided by the third MOS transistor 130 and thegeometric ratio of the first MOS transistor 121 and the second MOStransistor 122, is not dependent on the supply voltage VDD or VSS. Sucha current is also referred to as PTAT current (PTAT: “Proportional ToAbsolute Temperature”).

In the third current path 103, a fourth MOS transistor 140 of then-channel type is provided, via which the MOS transistor 113 isconnected to VSS. The fourth MOS transistor 140 is operated with thesame gate voltage Vb as the third MOS transistor 130 and in the regionof strong inversion and in saturation. This results in a quadraticdependence of the current Iptat through the fourth MOS transistor 140 onthe gate voltage Vb. On account of the saturation of the fourth MOStransistor 140, a dependence of the current Iptat through the fourth MOStransistor 140 on VDD and VSS is negligible. For generating the gatevoltage Vb, the gate terminal of the fourth MOS transistor 140 isconnected to the drain terminal of the fourth MOS transistor 140. Whatis achieved thereby in connection with the quadratic characteristicmentioned is that the gate voltage Vb is established depending on thecurrent Iptat, wherein influences of the supply voltages VDD and VSS arenegligible.

The fourth MOS transistor 140 can be dimensioned in a manner deviatingfrom the third MOS transistor 130. By way of example, the fourth MOStransistor 140 can have a smaller channel width than the third MOStransistor 130. The ratio of the channel widths between the third MOStransistor 130 and the fourth MOS transistor 140 may be e.g. 5:4. Theresistance value provided by the third MOS transistor 130 can beselected by means of this geometric ratio.

By virtue of the operation of the fourth MOS transistor 140 in stronginversion and saturation, while the current Iptat through the thirdcurrent path 103 is determined by means of the exponentialcharacteristic of the second MOS transistor 122, a stable setting of thegate voltage Vb and thus of the resistance value provided by the thirdMOS transistor 130 can be carried out.

The principle of matching the exponential characteristic of the secondMOS transistor 122 with the quadratic characteristic of the fourth MOStransistor 140 is illustrated in FIG. 3. In FIG. 3, the exponentialprofile—crucial for the second MOS transistor 122—of the drain-sourcecurrent Ids as a function of the gate voltage Vg is designated by A, andthe quadratic profile—crucial for the fourth MOS transistor 140—of thedrain-source current Ids as a function of the gate voltage Vg isdesignated by B. It is evident in FIG. 3 that an unambiguously definedoperating point for the gate voltages Vngate and Vb is defined by forthe same current Iptat through the second MOS transistor 122 and thefourth MOS transistor 140.

The oscillator circuit illustrated can be modified with regard to themodes of operation illustrated above. In this regard, by way of example,the first MOS transistor 121 and/or the second MOS transistor 122 couldbe operated in moderate inversion, thus furthermore resulting in adependence of the profile of the drain-source current which deviatesfrom the quadratic profile in the case of the fourth MOS transistor 140and thus enables a reliable setting of the operating point for the gatevoltages Vngate and Vb.

FIG. 2 further illustrates the generation of the reference voltage Vptatby means of the MOS transistor 150 provided in the current path 105. TheMOS transistor 150 is operated with the same gate voltage Vb as thethird MOS transistor 130 and the fourth MOS transistor 140, in stronginversion and in the linear region. The resistance value provided by theMOS transistor 150 is determined by the gate voltage Vb and can be inthe range of more than 1 MΩ, for example 10 MΩ. The MOS transistor 150can be dimensioned in a manner deviating from the third MOS transistor130 and/or the fourth MOS transistor 140. By way of example, the MOStransistor 150 can have a greater channel width than the fourth MOStransistor 140. The ratio of the channel widths between the MOStransistor 140 and the MOS transistor 150 can be e.g. 4:5, that is tosay that the MOS transistor 150 can be dimensioned in a manner similarto the third MOS transistor 130, but in a manner deviating from thefourth MOS transistor 140. The resistance value provided by the MOStransistor 150 can be selected by means of the geometric ratio to thefourth MOS transistor 140.

As shown in FIG. 2, in some implementations a further MOS transistor 160can be provided, which is coupled between the supply voltage line forVDD and a further supply voltage line for a further supply voltageVDDext. The MOS transistor may be, in particular, a high-voltagedepletion-mode MOS transistor. By means of the MOS transistor 160, thesupply voltage VDD can be derived from the further supply voltage. Byway of example, the further supply voltage could be in the range of 1 Vto 4 V or up to 70 V and a stabilization of the supply voltage VDD atapproximately 1.4 V could be achieved by means of the MOS transistor160. Dynamic currents in the oscillator circuit and in further circuitparts that use the oscillator signal OSC can be reduced by the MOStransistor 160. Furthermore, an improved electromagnetic compatibilitycan also be achieved, e.g. with regard to external interference signalscoupled in via the further supply voltage line.

A current path for discharging the capacitor 20 is not illustrated inFIG. 2. It goes without saying, however, that this current path can berealized in accordance with the current path 104, wherein in this casethe current Iptat is mirrored into the current path for discharging thecapacitor via a current mirror, based on MOS transistors of then-channel type, on the side of the supply voltage VSS.

For the resistance value provided by the third MOS transistor 130, insome cases there may be dependence on process variations occurringduring the production of the third MOS transistor 130 and/or fourth MOStransistor 140. Such process variations can influence for example a gateoxide capacitance of the MOS transistor 130 and/or 140. In particular,the resistance value provided can decrease as a result of a higher gateoxide capacitance and the resistance value provided can increase as aresult of a lower gate oxide capacitance. This would in turn result in ahigher current Iptat and a higher frequency of the oscillator signalOSC, and respectively a lower current Iptat and a lower frequency of theoscillator signal OSC. However, this circumstance can be taken intoaccount by the MOS transistor 25 of the capacitor being produced atleast partly by means of the same processes as the third MOS transistor130 and typically also the fourth MOS transistor 140. What can thus beachieved, for example, is that variations of the gate oxide capacitanceof the MOS transistor 130 and/or 140 also occur in a comparable mannerin the case of the MOS transistor 25 of the capacitor 20. In the lattercase, however, there is an oppositely directed dependence of thefrequency of the oscillator signal OSC. Specifically, in this case, ahigher gate oxide capacitance of the MOS transistor 25 typically bringsabout a lower frequency of the oscillator signal OSC, whereas a lowergate oxide capacitance of the MOS transistor 25 typically brings about ahigher frequency of the oscillator signal OSC. Production of the thirdMOS transistor 130 and/or of the fourth MOS transistor 140 by and alsoof the MOS transistor 25 by common processes can thus bring about acompensation of the effects of process variations.

For comparable reasons, the MOS transistor 150 for generating thereference voltage Vptat can also be produced at least partly by the sameprocesses as the MOS transistor 25 of the capacitor 20. In this case, byway of example, a higher gate oxide capacitance of the transistor 150can bring about a lower resistance value provided, a lower referencevoltage Vptat and thus a higher frequency of the oscillator signal OSC.Conversely, a lower gate oxide capacitance of the transistor 150 canbring about a higher resistance value provided, a higher referencevoltage Vptat and thus a lower frequency of the oscillator signal OSC.In this case, too, the effects of the process variations are thusdirected oppositely to those in the case of the MOS transistor 25.

FIG. 4 shows an exemplary profile of the current consumption of anoscillator circuit operating in accordance with the principlesillustrated above, as a function of the temperature. In this case, aleakage current of the entire integrated circuit was excluded frommeasurements. It is evident that at room temperature, but also at highertemperatures, the current consumption is significantly less than 100 nA.Even at temperatures of above 150° C., a current consumption of 100 nAis not exceeded.

FIG. 5 shows an exemplary profile of the frequency of an oscillatorsignal generated by an oscillator circuit operating in accordance withthe principles illustrated above, as a function of the temperature. Itis evident that the frequency is stable over a wide temperature range.In order to minimize the temperature dependence, an adaptation of thesaturation voltage of the first MOS transistor 121, of the second MOStransistor 122 and/or of the fourth MOS transistor 140 can also beperformed.

FIG. 6 shows a flow diagram for elucidating a method for operating anoscillator circuit, e.g. the oscillator circuit explained with referenceto FIGS. 1 to 3.

In step 610 a first field effect transistor is operated in saturation onthe basis of a first gate voltage. The first field effect transistor issituated in a first current path. By way of example, the first fieldeffect transistor can be the MOS transistor 121 in the current path 101.The first field effect transistor can be operated in weak inversion andsaturation in particular on the basis of the first gate voltage.

In step 620 a second field effect transistor is operated in weakinversion and in saturation on the basis of the first gate voltage. Thesecond field effect transistor is situated in a second current path andis dimensioned in a manner deviating from the first field effecttransistor. By way of example, the second field effect transistor can bethe MOS transistor 122 in the current path 101. The second field effecttransistor can be operated in weak inversion and saturation inparticular on the basis of the first gate voltage.

In step 630 a third field effect transistor is operated in stronginversion and in the linear region on the basis of a second gatevoltage. The third field effect transistor is situated in the secondcurrent path and is connected in series with the second field effecttransistor. By way of example, the third field effect transistor can bethe MOS transistor 130 in the current path 102.

In step 640 a fourth field effect transistor is operated in stronginversion and in saturation on the basis of the second gate voltage. Thefourth field effect transistor is situated in a third current path. Byway of example, the fourth field effect transistor can be the MOStransistor 140 in the current path 103.

In step 650 a current in the second current path is mirrored into thefirst current path, the third current path and at least one current pathof a multivibrator of the oscillator circuit. By way of example, acapacitor of the multivibrator, e.g. the capacitor 20, can be chargedvia the at least one current path of the multivibrator. Furthermore, areference voltage for a comparator of the multivibrator, e.g. thereference voltage Vptat, can be generated via the at least one currentpath of the multivibrator.

It goes without saying that in the method in FIG. 6 steps 610, 620, 630,640, 650 do not have to be performed successively in the orderillustrated. Rather, the steps can also be carried out substantiallysimultaneously.

Furthermore, it goes without saying that in the case of the exemplaryembodiments illustrated diverse modifications are possible, withoutdeparting from the basic idea of the concepts illustrated. By way ofexample, the concepts illustrated could be used in connection withdifferent types of integrated circuits. Furthermore, not only MOStransistors but also other types of field effect transistors can beused. Furthermore, it is noted that in some implementations the currentIptat need not necessarily be mirrored into the other current paths inthe ratio of 1:1, rather deviating ratios can also be used. Differenttypes of multivibrators can also be used, e.g. a multivibrator having aswitchable or otherwise settable frequency of the oscillator signalgenerated. By way of example, the oscillator circuit described abovecould be switchable into a mode of operation in which the oscillatorsignal OSC is generated with increased frequency, e.g. if the integratedcircuit that uses the oscillator signal is transferred from energysaving operation to normal operation.

The invention claimed is:
 1. Oscillator circuit, comprising: amultivibrator for generating an oscillator signal, wherein themultivibrator comprises at least one current path; a supply circuithaving a first current path, a second current path and a third currentpath; a current mirror for mirroring a current through the secondcurrent path into the first current path, the third current path and theat least one current path of the multivibrator; a first field effecttransistor in the first current path, wherein the first field effecttransistor is configured to be operated in saturation on the basis of afirst gate voltage; a second field effect transistor in the secondcurrent path, which is dimensioned in a manner deviating from the firstfield effect transistor, wherein the second field effect transistor isconfigured to be operated in saturation on the basis of the first gatevoltage; a third field effect transistor in the second current path,wherein the third field effect transistor is configured to be operatedin strong inversion and in the linear region on the basis of a secondgate voltage; and a fourth field effect transistor in the third currentpath, wherein the fourth field effect transistor is configured to beoperated in strong inversion and in saturation on the basis of thesecond gate voltage.
 2. Oscillator circuit according to claim 1, whereinthe first field effect transistor is configured to be operated in weakinversion and saturation on the basis of the first gate voltage. 3.Oscillator circuit according to claim 1, wherein the second field effecttransistor is configured to be operated in weak inversion and saturationon the basis of the first gate voltage.
 4. Oscillator circuit accordingto claim 1 wherein the at least one current path of the multivibratorcomprises a fourth current path for at least one of charging anddischarging a capacitor of the multivibrator.
 5. Oscillator circuitaccording to claim 4, wherein the capacitor is embodied on the basis ofa fifth field effect transistor.
 6. Oscillator circuit according toclaim 5, wherein the third field effect transistor and the fifth fieldeffect transistor are embodied at least partly by common processes. 7.Oscillator circuit according to claim 6, wherein the at least onecurrent path of the multivibrator comprises a fifth current path forgenerating a reference voltage for a comparator of the multivibrator. 8.Oscillator circuit according to claim 7, wherein the fifth current pathcomprises a sixth field effect transistor, which is configured to beoperated in the linear region on the basis of the second gate voltage.9. Oscillator circuit according to claim 8, wherein the fifth fieldeffect transistor and the sixth field effect transistor are embodied atleast partly by common processes.
 10. Oscillator circuit according toclaim 8, wherein the comparator is embodied on the basis of a singlefield effect transistor.
 11. Oscillator circuit according to claim 1,further comprising: a high-voltage depletion-mode field effecttransistor coupled between a first supply voltage line for providing asupply voltage for the at least one current path of the multivibratorand for the first, second and third current paths of the supply circuitand a further supply voltage line.
 12. Oscillator circuit according toclaim 1, wherein at room temperature a current consumption of theoscillator circuit is less than 100 nA.
 13. Integrated circuitcomprising an oscillator circuit, wherein the oscillator circuitcomprises: a multivibrator for generating an oscillator signal, whereinthe multivibrator comprises at least one current path; a supply circuithaving a first current path, a second current path and a third currentpath; a current mirror for mirroring a current through the secondcurrent path into the first current path, the third current path and theat least one current path of the multivibrator; a first field effecttransistor in the first current path, wherein the first field effecttransistor is configured to be operated in saturation on the basis of afirst gate voltage; a second field effect transistor in the secondcurrent path, which is dimensioned in a manner deviating from the firstfield effect transistor, wherein the second field effect transistor isconfigured to be operated in saturation on the basis of the first gatevoltage; a third field effect transistor in the second current path,wherein the third field effect transistor is configured to be operatedin strong inversion and in the linear region on the basis of a secondgate voltage; and a fourth field effect transistor in the third currentpath, wherein the fourth field effect transistor is configured to beoperated in strong inversion and in saturation on the basis of thesecond gate voltage.
 14. Method for operating an oscillator circuit,comprising: operating a first field effect transistor in a first currentpath in weak inversion and saturation on the basis of a first gatevoltage; operating a second field effect transistor in the secondcurrent path in weak inversion and saturation on the basis of the firstgate voltage, wherein the second field effect transistor is dimensionedin a manner deviating from the first field effect transistor; operatinga third field effect transistor in the second current path in stronginversion and in the linear region on the basis of a second gatevoltage; operating a fourth field effect transistor in a third currentpath in strong inversion and in saturation on the basis of the secondgate voltage; and mirroring a current in the second current path intothe first current path, the third current path and into at least onecurrent path of a multivibrator for generating an oscillator signal. 15.Method according to claim 14, wherein the first field effect transistoris a metal-oxide-semiconductor (MOS) transistor and is operated in weakinversion and saturation on the basis of the first gate voltage. 16.Method according to claim 14, wherein the second field effect transistoris a metal-oxide-semiconductor (MOS) transistor and is operated in weakinversion and saturation on the basis of the first gate voltage. 17.Method according to claim 14, comprising: charging a capacitor of themultivibrator via the at least one current path of the multivibrator.18. Method according to claim 14, comprising: generating a referencevoltage for a comparator of the multivibrator via the at least onecurrent path of the multivibrator.
 19. Method according to claim 14,comprising: generating the oscillator signal, wherein the oscillatorcircuit comprises the multivibrator, a supply circuit having the firstcurrent path, the second current path and the third current path, acurrent mirror for mirroring the current through the second current pathinto the first current path, the third current path and the at least onecurrent path of the multivibrator, the first field effect transistor,the second field effect transistor, the third field effect transistor,and the fourth field effect transistor.